1. Field of the Invention
This invention relates to the field of short-range radar sensors on land and water used to protect assets against unwarranted intruders and terrorists.
2. Description of Prior Art
There is a continual requirement to protect a given perimeter over wide angles from approaching intruders. This is often to protect military assets but also can be used for home and industry as well. Intruders may take the form of walkers or crawlers or runners or even vehicles. For shipboard application, intruders may approach a docked ship in a harbor under a variety of sea state conditions. Sensors currently used to provide perimeter detection employ a narrow microwave beam breaker system similar to a photoelectric device. The microwave beam breaker is subject to high levels of false alarms especially under varying weather conditions; the photoelectric system is, generally, limited in range. Both the photo-electric and microwave beam sensors cannot look beyond, for example, a fence; an alarm is sounded only when the beam barrier is crossed.
Ultrasonic sensors provide an ability to look out beyond the fence or barrier. Their range, however, is limited to less than 100 feet. Some sonic sensors have ranges beyond 100 feet, but cannot resolve multiple targets. Optical cameras, coupled with infrared night-vision and motion-sensing capabilities, can only view narrow azimuths, and are currently very expensive to implement. Radar sensors, as described in references j and k, appear to have the best properties for establishing wide-angle coverage over a perimeter over hundreds of feet in front of a fence or barrier without clutter while establishing a sufficient resolution to detect human and vehicle targets. However they are subject to false detections because of clutter.
In uncluttered areas, CW-Doppler, FM-CW and Ultra-Wideband (UWB) short-pulse radar sensors have been used to detect and resolve walking and crawling human targets over ranges measured in hundreds of feet. The limits on detecting crawling targets depend on the height and frequency of the radar sensor because of likely signal cancellation due to ground bounce. Besides being a very low radar cross-section target, the radar response from a crawler close to the ground is especially sensitive to ground bounce cancellation. This is because the differential path lengths between the direct and ground bounce signals are very small. It can be shown that the differential path length, .DELTA., between the direct wave and that of the ground bounce signal is given by: .DELTA.=2hH/r, where h is the height above ground of the crawler (e.g., 1 foot), H is the height of the radar antenna above the ground, and r is the distance to the crawler. It can be seen that for distances as close as 150 feet, and a radar height of 10 feet, the differential path length before cancellation begins is only .DELTA.=20/150 feet or 0.133 feet. And since the speed of light travels one foot in 1 nanosecond, this translates to a time delay difference of but 133 picoseconds. If the ground is wet, the reflection coefficient can be as high as 0.8. For these reasons, it can be shown that a short pulse UWB radar with a chosen nominal center frequency whose period is four times 133 ps or less will permit detection of crawlers within the first quarter cycle before cancellation begins. This is described in references c and d.
A prime problem is to somehow accommodate, automatically, the varying amounts of ground or sea clutter present within the radar field of view, and still achieve detection while maintaining a high probability of detection and a low probability of false alarm. The subject invention addresses and solves these issues.